The present invention relates generally to a method for heat treating and sintering oxide materials with microwave radiation, and more particularly to the sintering of such materials which do not function effectively as microwave coupling materials at relatively low temperatures. The oxide material is enclosed in a housing which is formed of a material characterized by microwave coupling capabilities at lower temperatures for pumping the encased oxide material to a sufficient temperature to enable the oxide material to couple with the microwave radiation and then provide thermal insulation for retaining the heat in the oxide material.
Microwave radiation is becoming of increasing importance as a very efficient and effective tool for the heat treatment and sintering of materials such as refractory metal oxides. Microwave radiation very efficiently and rapidly heats the oxide materials to the relatively high temperatures required for sintering. Also, the microwave radiation penetrates the particulate mass of oxide material to providing a more uniform sintering of the oxide material than heretofore realized with the relatively non-uniform heat up provided by the conventional furnaces. However, in spite of many advantages it has been found that there are some problems attendant with the use of microwave radiation for the sintering of many oxide materials due to their being essentially transparent or non-coupling with microwave radiation even at relatively high frequencies of up to about 2.45 GHz at temperatures in a range of about room temperature up to about 900.degree. C.
In order to effect the sintering of such oxide materials with microwave radiation, materials capable of absorbing or coupling with microwave radiation such as boron carbide, silicon boride, titanium nitride, or uranium oxide were previously admixed with the particulate oxide material for permitting the sintering of the oxide material with microwave radiation. In these instances the coupling materials function to heat the surrounding particulates of oxide material with microwave radiation with the heat being transferred from the coupling material to the surrounding oxide particulates. This heating continues until the oxide material reaches a temperature of several hundred degrees centrigrade where high frequency relaxation mechanisms such as ionic conduction and molecular vibrations occur in the oxide material. These relaxation mechanisms cause the oxide material to couple or absorb microwave radiation so that the oxide material can be heated thereby to the desired sintering temperature.
While the addition of coupling material to the body of oxide particulates forming a compact to be sintered has proven to be useful for the microwave heating of oxide materials from room temperature up to the temperature where the oxide materials couple with the microwave radiation, the purity of the sintered oxide material may be compromised by the presence of the added coupling material. Further, the properties and characteristics of the sintered oxide material with this added impurity are often altered sometimes deleteriously.